A first conventional optical amplification system comprises a pump light source for emitting a pump light having a wavelength of, for instance, 0.98 .mu.m, an optical coupler for coupling a signal light having a wavelength of, for instance, 1.55 .mu.m and the pump light to provide a coupled light, and an Er(erbium)-doped optical fiber for amplifying the signal light, as described on pages 551 to 553 of "IEEE PHOTONICS TECHNOLOGY LETTER, VOL. 3, No. 6, June, 1991".
In the Er-doped optical fiber, the signal light included in the coupled light is amplified in accordance with the pumping with the pump light. Thus, the amplified signal light is obtained therein to be supplied to a following stage.
However, an amplified spontaneous emission (ASE) light having arbitrary polarizations is also generated in the Er-doped optical fiber, so that the ASE light will be noise at the following stage. Further, a ASE light beat noise generated among wavelengths of the ASE light N.sub.ASE--ASE is added to the output light of the Er-doped optical fiber. The ASE light beat noise N.sub.ASE--ASE is defined by the equation. EQU N.sub.ASE--ASE =m.sub.t .multidot.(G-1).sup.2 .multidot.n.sub.sp.sup.2 .multidot..DELTA..upsilon. (1)
where m.sub.t is a transverse mode degree, G is an amplification gain of the Er-doped optical fiber, n.sub.sp is a population distribution parameter of an amplification medium, and .DELTA..upsilon. is the ASE light bandwidth. Therefore, the ASE light is suppressed by controlling the population distribution parameter of Er atoms to be, for instance, 1. Consequently, low noise amplification is realized in the first conventional optical amplification system.
A second conventional optical amplification system comprises a pump light source, an optical coupler, an Er-doped optical fiber, and an optical filter which is connected to an output of the Er-doped optical fiber as described in pages 288 to 297 of "INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS OF JAPAN PAPERS B-I, VOL. J75-B-I, No. 5".
In the optical filter, ASE lights generated in the Er-doped fiber having wavelengths equal and proximate a wavelength of a signal light pass, while ASE lights having other wavelengths are removed therein. Consequently, a low noise amplification system is realized in the second conventional optical amplification system. The similar systems are described in Japanese Kokai No. 1-298785, No. 1-127886 and No. 1-152819.
In the first conventional optical amplification system, however, there is a disadvantage in that ASE lights inside the signal light band can not be reduced, because a band-pass optical filter is used.
Further, the second conventional optical amplification system has a disadvantage in that the transverse mode degree m.sub.t of the ASE light is equivalently to be 2 to result in the generation of beat noise which is surplus by times of the transverse mode degree m.sub.t, because polarizations of the ASE light generated in the Er-doped optical fiber are random.